Vestager also said she was wary of too many mergers in the telecoms sector, warning that consumers could end up with higher bills and less innovative companies.

"Incumbent operators argue that if they cannot merge with their rivals in the same country they will be unable to increase their investment. I've heard this claim quite often, but I have not seen evidence that this is the case," she said.

"Instead, there is ample evidence that excessive consolidation may lead not only to less competition and more expensive bills for consumers, but that it also reduces the incentives in national markets to innovate."

Vestager's comments come as she scrutinizes a proposal by Norwegian telecoms company Telenor and Swedish peer TeliaSonera to combine their Danish mobile businesses.

She has already expressed concerns the deal may lead to higher prices.

A bigger deal is expected to land on her desk in the coming months: Hutchison Whampoa's bid to become Britain's biggest operator by buying Telefonica's British business, O2 UK, for 10.3 billion pounds ($16 billion).